Fungicide sensitivity of Colletotrichum acutatum from Australian olives

Abstract: Olive anthracnose is primarily caused by Colletotrichum acutatum to the major olive growing region along the Murray River valley of Australia. Significant crop losses occur in seasons with abnormally wet summer conditions with some disease carry-over into subsequent years. This was the case in 2010-11 and again in 2022-23. Field trials were conducted in 2011-12 along with detached fruit assays which determined that the QoI fungicides, azoxystrobin and pyraclostrobin had superior relative efficacy compared with several copper formulations. In 2023, 12 isolates from that earlier study along with 75 fungal isolates from the 2022-23 season were used in in-vitro assays to determine if there had been a shift in sensitivity to azoxystrobin. All isolates had an EC50 of less than 1 μg/ml suggesting that no significant resistance had developed, in contrast to reports of highly resistant populations of C. acutatum from strawberries in Florida where EC50 values exceed 100 μg/ml. Azoxystrobin formulations are currently registered for use on olives in Australia at an equivalent spraying concentration of 200 μg a.i./ml, so the current sensitivity of C. acutatum isolates demonstrated in this study is well within that limit. Unfortunately, there were no QoI naïve isolates available for this study, so it remains possible that some decreased sensitivity had developed prior to their first use in the mid-2000s.